In cellular telecommunication systems a single speech connection or data connection through the cellular telecommunication network is called a bearer. Generally, a bearer is associated with a set of parameters pertaining to data communication between a certain terminal equipment and a network element, such as a base station or an interworking unit (IWU) connecting the cellular network to another telecommunications network. The set of parameters associated with a bearer comprises typically for example data transmission speed, allowed delays, allowed bit error rate (BER), and the minimum and maximum values for these parameters. A bearer may further be a packet transmission bearer or a circuit switched bearer and support for example transparent or non-transparent connections. A bearer can be thought of as a data transmission path having the specified parameters connecting a certain mobile terminal and a certain network element for transmission of payload information. One bearer always connects only one mobile terminal to one network element. However, a bearer can pass through a number of network elements. One mobile communication means (ME, Mobile Equipment) may in some cellular telecommunication systems support one bearer only, in some other systems also more than one simultaneous bearers.
In order to be able to transmit information in a desired way, connections over the radio interface have to obtain a desired level of quality. The quality can be expressed for example as the C/I i.e. Carrier to Interference ratio, which indicates the ratio of received carrier wave power to received interfering power. Other measures for the quality of a connection are SIR i.e. Signal to Interference ratio, S/N i.e. Signal to Noise ratio, and S/(I+N) i.e. Signal to Noise plus Interference ratio. The bit error rate (BER) or frame error rate (FER) are also used as measures of connection quality. Typically, a certain target level for one of these or other corresponding measures is determined beforehand, and for each connection, the transmission power is adjusted to be such that the target level is reached as closely as possible. The transmission power should not be higher than what is necessary for obtaining the desired target level, since a too high transmission level wastes electrical energy in the transmitting equipment, which is crucial with handheld mobile stations, and causes interference to other connections.
Admission control is a crucial function in ensuring, that each bearer obtains the desired SIR level. The purpose of admission control is to examine each new request for a new bearer, and determine whether the requested service can be provided without degrading the service to other bearers, taking into account the transmission power of the requested bearer. If the new bearer can be serviced without harming other bearers, the request is admitted. Admission control typically co-operates with power control, whereby the transmission power of some of the other bearers may be adjusted in order to guarantee the SIR target level of the other bearers.
Various admission control algorithms have been proposed in the past. The article “SIR-Based Call Admission Control for DS-CDMA Cellular Systems” by Zhao Liu and Magda El Zarki, I2 Journal on selected areas in communications, vol. 12, no. 4, pp. 638–644, May 1994, describes an algorithm based on the concept of residual capacity. Residual capacity is defined as the additional number of initial calls a base station can accept. If the residual capacity is larger than zero, new calls are admitted. The residual capacity is determined from measured SIR levels and a treshold SIR level.
Another algorithms are described in the article “Call Admission in Power Controlled CDMA Systems” by Ching Yao Huang and Roy D. Yates, in proceedings of I2 VTS 46th Vehicular Technology Conference, Apr. 28–May 1, 1996, Atlanta, USA, pp. 1665–1669. In this article, two simple algorithms are presented. In the first algorithm, a new call is blocked when that new call would cause ongoing calls to transmit at maximum power. In the second algorithm, a new call is blocked if the total received power measured at the base station exceeds a predetermined treshold.
These algorithms function well, when the calls i.e. bearers are relatively similar in terms of resource usage, and any admission tresholds are set to a level where the admission of a bearer does not increase the load too near to the maximum capacity. However, these algorithms do not function well, when the bearers have widely varying properties, i.e. when the network needs to handle both low bit rate bearers such as normal speech bearers, and high bit rate bearers such as high-capacity data bearers or live video bearers. Such a variety of services will be provided for example by the UMTS cellular telecommunication system presently under development. For example, in the conventional algorithm in which a new call is allowed if the total received power measured at the base station is under a predetermined treshold, a high bit rate bearer may increase the network load too near to the maximum capacity. This can be prevented by lowering the treshold so that any high rate bearers allowed close to the treshold still do not increase the total load too much, but in that case, the low bit rate speech bearers end up being refused even if the remaining capacity could accommodate them.
The problem with the known admission control methods is, that they consider all bearers to have an evenly distributed bit rate usage, without regarding the differing properties of different bearers. This presents a problem in a situation, where there are bearers with widely differing characteristics within a control region, for example a large number of voice calls and a few real time video connections. A problem with high bit rate bearers is that even though the received total power in their control region is quite satisfactory, that control region may cause too much interference to some neighbouring control region because of the unequal distribution of the interfering power within it.